1. Field of the Invention
The invention concerns a method of purifying smoke from a livid fuel (in particular heavy fuel oil) boiler by eliminating unwanted components including acids in which method the smoke, before it is exhausted to the atmosphere, is caused to pass along a vertical path in a heat exchanger adapted to recover the latent heat of condensible components together with the substantive heat of the smoke and water is sprayed at the top of said vertical path and collected at the bottom thereof in the form of an effluent comprising said water, condensates and soot.
2. Description of the Prior Art
There is no need to expatiate on the consequences of exhausting to the atmosphere smoke from liquid fuel boilers, such liquid fuel essentially comprising hydrocarbons. The smoke contains mainly water vapor, carbon monoxide, carbon dioxide and nitrogen from the combustion-supporting air. To give an idea of the orders of magnitude involved, combustion of 1 kg of hydrocarbon requires approximately 17.5 kg of and the smoke contains, assuming that all of the carbon is converted into carbon dioxide, approximately 14 kg of nitrogen, 3.14 kg of carbon dioxide, 1.28 kg of water vapor and less than 0.5 kg of oxygen.
The combustion of hydrocarbons produces various pollutants: at the temperature of the flame the reaction between the oxygen and nitrogen in the combustion-supporting air produces nitrogen oxides and the reaction between the sulfur and sulfur compounds in the hydrocarbons produces gaseous sulfur compounds, essentially sulfur dioxide and sulfur trioxide; the hydrocarbons contain trace amounts of many elements including heavy metals in particular; finally, the combustion of carbon-rich hydrocarbons produces soot.
Note that the quantities of nitrogen oxides and carbon monoxide depend on the condition of the flame. Although it is possible to reduce the amount of carbon monoxide in the smoke, for example by injecting secondary air, it is not possible in practise to reduce the amount of nitrogen oxides once formed. Turning to a separate issue, although exhausting water vapor into the atmosphere does not constitute pollution as such, the water vapor retains its latent heat of condensation. Also, on condensing in contact with the atmosphere, especially on leaving the chimney (to forth the wreath), it carries with it to ground level some of the pollutants which accompany it in the smoke; furthermore, the energy wasted as latent heat is reflected in excess fuel consumption.
It has been proposed to pass the smoke along a vertical path in a heat exchanger, to wash the smoke by spraying it with water at the top of the heat exchanger and to collect this water at the bottom of the heat exchanger in the forth of an effluent containing condensates and soot. Cooling the smoke recovers the latent heat of the condensates (which may be used to heat the boiler feed water). Additionally, the effluent contains dissolved carbon dioxide, sulfur dioxide and sulfur trioxide removed from the smoke.
This proves to be insufficient, however, given the limited solubility of carbon dioxide and sulfur dioxide in water at the necessarily high temperature of the effluent. Also, sulfur trioxide is only ever a minor proportion of the sulfur oxides and dissolving it in water further reduces the solubility of the carbon dioxide and sulfur dioxide.
Above all, the effluent collected at the bottom of the heat exchanger must be treated before it can be discharged into the environment. Eliminating the soot raises particular problems given that the behavior of suspensions is highly dependent on the composition of the suspension medium.